US2895838A - Metal casting mold material - Google Patents

Description

lVIETAL CASTlNG MOLD MATERIAL Frank P. Ilenda, Painesville, Ohio, assignor to Diamond Alkali Company, Cleveland, Ohio, a corporahon of Delaware No Drawing. Application September 5, 1956 Serial No. 608,000

11 Claims. (Cl. 106-383) This invention relates to improvements in W silicate-containing mo s a apte ened by impregnation with carbon dioxide or other gaseous curing agent to molds formed therefrom and to methods for their preparation and use.

In recent years there has been considerable interest in the preparation of molds and cores from sand-silicate mixtures which are cured or hardened in the desired casting-defining shapes by exposure to carbon dioxide or other gaseous curing materials. Generally, the curing of molds embodying binders of this type involves a chemical reaction between carbon dioxide or other gaseous material and the alkali metal silicate dispersed throughout the sand or other refractory material substantially completely to convert the alkali metal silicate into a uniformly-dispersed silicon dioxide gel binder and an alkali metal carbonate.

The advantages of such molds are many. Not only are aging and post-curing treatments eliminated, thus permitting rapid fabrication of molds without requiring the use of expensive equipment, but also close tolerance castings are obtainable. Moreover, the process is relatively simple and can be carried out readily on a large scale.

However, despite the many advantages of such molds, it frequently has been found that certain ditficulties are encountered with prior silicate-containing mold-forming compositions of this type at casting temperatures generally, and especially in the casting of high temperature melting metals, such as cast iron and steels melting at temperatures of 2500 F. or higher.

At casting temperatures, prior silicate-containing moldforming compositions generally have been characterized by excessive evolution of steam and/ or other gases during casting, if sufficient organic additive was present to provide satisfactory collapsibility.

Another problem which has heretofore been especially troublesome was that of removing the mold-forming composition from the castings after solidification of the metal. Up to the time of the inventions described and claimed in co-pending applications Ser. No. 549,850, filed November 29, 1955; Ser. No. 540,633, filed October 14, 1955; and Ser. No. 568,442, filed February 29, 1956, there was no generally useful solution known to the problem of providing adequate collapsibility of the mold after solidification of metal therein whereby the moldforming composition is readily removable from the casting.

The present invention comprises an improvement over the prior inventions described and claimed in the above-identified applications especially with respect to improvements in the collapsibility or so-called fired strength of alkali metal silicate-containing mold-forming compositions.

Accordingly, it is the principal object of the present invention to provide a new and improved alkali metal silicate-containing mold-forming composition which is CROSS RtttHLNut LAFHIHIu-l! 2,895,838 Patented July 21, 1959 characterized by a singular improvement in mold collapsibility or fired strength.

A further object of the invention is to provide new and improved compositions for forming alkali metal silicate- 5 containing molds adapted to be cured by exposure to carbon dioxide or other substances.

A still further object of the invention is to provide new and improved methods of metal casting.

A still further object of the invention is to provide an improved shell mold and methods of shell molding. These and other objects and advantages of the invention will appear more fully from the following description thereof.

As used throughout the specification and claims, the term mold is intended in a generic sense to mean casting forms which include both molds and cores, this invention in no manner being limited to the former. Moreover, mold is intended to include various patterns for use in the casting art, as well as shell molds including shell mold-forming elements in addition'to a completed shell mold structure prepared by assembling two or more complementary thin-walled shell mold elements. Hence, it will be appreciated that the term mold is used to include a casting-defining surface broadly.

The present invention contemplates as a mold-forming composition a mix tgrepimammfii rnptq ortionof .afinelyrL refractory ater al.amino:. pr.oponion-n.an alkali metafl filieafe'and a min r proportion lnfhatl least 'ofiephbsphateiieif'a"salt'or e ster of anoxygen acid of fifimfiis'ifgiiding 5Ttidffbf orthophosphoric v E). This invention also contemplates as a novl'c'dmposition of matter a mixture of a major proportion of a finely-divided refractory material,

e.g., sand, and a minor proportiqnof sueh aphosphate.

composition which can be cured readily by carbon dioxide or other curing agent to form a rigid mold having an excellent unfired or green strength and yet which after firing and solidification of a casting therein, is characterized by such a low fired strength as to permit its ready removal from the casting surfaces.

The phosphate employed generally should be an alkali phosphate, the term alkali phosphate being intended to include both alkali metal phosphates and alkaline earth metal phosphates as well as ammonium phosphates. Thus, it will be appreciated that phosphates of the alkaline earth metals generally are contemplated such as calcium phosphates, magnesium phosphates, beryllium fi plelsss rmimr osph t a.bfi ifiiil i fidium phosphates, although calcium phosphates'cornprise the preferred type of alkaline, .earth metal phosphates.

0 "Itwill be further understood, of course, that the term alkali metal phosphates include, in addition to sodium phosphates, phosphates of lithium, potassium, rubidium and cesium. It will be further understood, of course, that of these materials, sodium phosphates and potassium phosphates represent the most readily available and practicable materials, it being specifically preferred that sodium phosphates be employed from the alkali metal phosphate group.

In many instances organic phosphates, too, are advantageous and are intended to be encompassed by the term phosphate. For example, triaryl phosphates, e.g., tricresyl phosphate and triphenyl phosphate, trialkyl phosphates, and the like, as well as diand mono-organic phosphates, may be used.

Illustrative of specific alkali phosphates of the foregoing types are calcium phosphates, such as CaHPO CaH (PO and Ca(PO tricalcium phosphate being preferred. Specific phosphates include metaphosphates, e.g., NaPO (NaPO KPO; and their hydrates; polyphosphates, e.g., Na P O and its hydrates; pyrophosphates, e.g., Na P O and its hydrates; orthophosphates, e.g., NaH POI, and its hydrates including NaH- PO -H O, Na' 'HPUZWI-I O, and Na PO,,-12H O; and alkali metal phosphates such as monoand di-alkali metal phosphates, e.g., monosodium phosphate and disodium phosphate, monopo assium p e and dipotassium phosphate, and ammonium phosphates, e.g., NH H PO and (NH,) HPO Of the foregoing types of phosphates, metaphosphates are preferred.

In practice, it generally is preferred at present to employ a phosphate in the form of a relatively inexpensive material such as one of the naturally occurring phosphatecontaining minerals such as are used as fertilizers and in other applications. Such materials provide the desired phosphate largely as a calcium phosphate and do not contain as non-phosphate ingredients substances which interfere with the formation of molds and casting of metals therein when this type of phosphate is employed in combination with an alkali metal silicate in accordance with the practice of this invention.

Illustrative of such low cost phosphate sources are the so-called ground phosphate rock substances such as that which contains about 68%-78% tricalcium phosphate and minor amounts of calcium fluoride, silica and oxides of iron and aluminum. A typical analysis of such a material is the following wherein the quantities are expressed in terms of percent by weight:

Percent Tricalcium phosphate 69-78 Calcium carbonate 2-5 Calcium fluoride O-3 Iron and aluminum oxides 1-4 Silica 5-10 Other finely-divided phosphate rock materials variously termed in the trade but generally characterized by containing a substantial proportion of available P also may be used. For example, those products containing apatite, (CaF)Ca (PO or CaF -3(Ca P O and the so-called bone phosphates may be used.

It is contemplated that silicates of the various alkali metals including potassium, sodium, cesium, rubidium and lithium may be employed, although sodium silicates and potassium silicates represent the more readily available silicates while sodium silicates clearly constitute the preferred and most practicable silicates. For that reason, special reference will be made herein to sodium silicates although it will be understood that the present invention is not limited thereto.

Generally, the term alkali metal silicate," preferably a odium silicate, may include silicates having varying silic em oxide ratios, e.g., those having a silica to alkali metal oxide ratio greater than l.0:1.0 and desirably within the range from about 1.5 to 3.8 generally being suitable, those silicates having a silica to alkali metal oxide ratio within the range from about 2.4 to 2.84 being preferred at present. It will be further understood that both liquid and dry silicates may be employed using, where necessary, sufficient water or other liquid to provide the desired silicate solution, slurry, plastic mixture or the like, depending upon the application intended.

More specifically, the present invention contemplates the use in the preparation of gas-curable molds of a binder containing an alkali metal silicate having an alkali metal oxide to silica ratio of from about 1.0:1.5 to 1.03.8, such as an Na O:SiO, ratio of 1.0:1.5 to 1.0:3.8.

The practice of the present invention contemplates as a. finely-divided refractory material refractory substances such as sand, e.g., Ottawa sand, finely-divided a l 14nina, zirconia, ground silica, silica flour, ground firebrick, ground magnesia firebrick, fused r n a gnesia, titanium oxide, beryllium oxide, mullite, sillimanite, as well as various mixtures thereof as well as other materials useful as refractory substances.

It will be observed that the practice of this invention does not require the use of an organic additive such as has heretofore been suggested for inclusion in a moldforming composition whereby it may be burned out during solidification of the metal, thereby to promote mold collapsibility. However, in many instances, while not necessary, the use of an organic additive insofar as providing mold collapsibility is concerned, is compatible with the practice of the present invention. In this connection, various polyhydroxy organic compounds such as those described and claimed in Ser. No. 540,633, filed October 14, 1955, may be employed.

While the proportions of phosphate and finely-divided refractory material can be varied, depending upon the metal being cast, the application intended, temperatures employed, and various other factors related to foundry practice, it generally is desirable to employ a major proportion of the finely-divided refractory material and only a minor proportion of the phosphate, this both on the grounds of economy and because only a minor proportion of the phosphate of this invention provides entirely adequate mold collapsibility. However, it generally has been found that the use of a mold-forming mixture comprising a phosphate in an amount up to about 5% by weight of the refractory material is desirable, generally amounts ranging from 0.5% to 2.5% by weight being preferred at present, 2% by weight being the specifically preferred amount. Optimum results are obtained when at least enough phosphate is added to react with the alkali metal carbonate formed on curing of the mold, e.g., with carbon dioxide, to form a tri-alkali metal phosphate instead of an alkali metal silicate.

With respect to the relative proportions of finely-divided refractory and silicate, it is preferable, of course, to employ a major proportion of the refractory material and a minor proportion of the silicate as a binder. In terms of the finely-divided refractory material used, it is typically desirable to employ a silicate in an amount up to about 15% by weight, smaller amounts within the range from about 3% to 6% by weight being preferred at present, the specifically preferred amount being 5% by weight.

In practice, the formation of a cured structure in accordance with the present invention comprises subjecting a mixture of a finely-divided, e.g., 60 to l00 mesh, refractory material containing a phosphate and a silicate binder to the action of carbon dioxide, carbon dioxidecontaining gases or other reactive substances capable of converting the mixture into a rigid, high strength material. Mixtures of these or other gaseous materials such as sulfur dioxide, nitrous oxide, hydrogen chloride, as well as various stack gases or other combustion gases, containing substantial amounts of carbon dioxide or other reactive acidic gases capable of effecting the desired curing of the silicate-containing mold-forming composition may be used.

In practice, the refractory material is blended with the hosphate, which pre era y 15 o t e same or er f mummy term binder being intended to refer to the various alkali metal silicates and alkali metal silicate-containing binder compositions. The blending may be done in any suitable manner, as by mulling, or other mixing operation.

The thus-prepared mixture is then placed in contact with a mold-defining surface such as a pattern. This can, if desired, be coated with a suitable parting or separating agent. When the refractory materialphosphate-binder mixture is in contact with the mold-defining surface, it

generally is desirable to compress the mixture slightly as by compacting it against the surface. The thus-compacted material is then impregnated or otherwise contacted with carbon dioxide or other suitable gaseous reactant as described hereinbefore for a short time to complete curing of the mold-forming composition.

Typically, carbon dioxide may be injected into a sandphosphate-sodium silicate binder mixture for a period of a few seconds up to one minute or more, depending on the size of the mold being prepared, the fineness of the sand, the proportions of the reactants and the like. After the gas-curing treatment, the mold is ready for immediate use without aging, heating or other curing treatments or processing.

The specific examples which follow are provided to illustrate the practice of the present invention and to demonstrate the excellent collapsibility attained by the low fired strength of molds embodying the present invention. In each instance, the unfired or green strength is sulficiently high to permit handling of the mold during mold assembly and other foundry operations without injury thereto.

The data indexed comparatively in the following examples is obtained by preparing 2" cubes from a mixture of Ottawa 60 grain sand, containing less than .25% water, and a phosphate compound in the indicating percent by weight of said sand, and as a binder a sodium silicate having a ratio of sodium oxide to silicon dioxide within the range from 1:2.2-2.6l which constitutes the especially preferred type of sodium silicate. The mold-forming composition is prepared by adding the phosphate compound to the sand and intimately mixing it therewith and then introducing the silicate to complete the mixture. The resultant cubes are then cured by impregnation with carbon dioxide for seconds at 20 p.s.i.g. at room temperature. The cubes are then sintered at the temperatures indicated for a period of five minutes, and subjected to compression loading using a Tinius Olsen compression testing machine.

forming composition after exposure to high temperatures.

While the present invention is of general utility in the metal casting arts, the practice of this invention also is useful in the shell molding art which employs thin-walled dispensable molds and cores generally formed of sand and a suitable binder. In the past, shell molding processes generally have utilized a thermosetting resinous binder and have required a heat curing step prior to casting. By the practice of the present invention, shell molding operations may be conducted using rigid, thinwalled molds having high gas permeability, good surface smoothness and dimensional stability formed in accordance with this invention by applying a layer of a mixture embodying the invention against a pattern. The thus-formed layer in contact with the pattern can then readily be subjected to the action of carbon dioxide or other gases as described hereinbefore. By such a process there is formed a shell type mold section accurately reproducing pattern details and having suflicient rigidity for use, hacked or unbacked. By employing complementary shell mold sections formed in this manner which include a casting-defining surface therebetween, the many advantages of the shell molding art can be realized without employing the expensive and often critical heat curing heretofore considered an essential feature of the shell molding operation.

It is to be understood that although the invention has been described with specific reference to particular embodiments thereof, it is not to be so limited, since changes and alterations therein may be made which are within the full intended scope of this invention as defined by the appended claims.

What is claimed is:

1. The method of forming a metal casting mold which comprises placing in contact with a casting-defining surface an intimate mixture of a major proportion of a finely-divided refractory material, a minor amount of a phosphate compound selected from the group consisting of alkali metal phosphates, alkaline earth metal phosphates,

Sodium Percent Silicate Fired Green Example Phosphate by Binder(5%) Strength Strength No. Weight NmOzSiO; (p.s.i.) .s.i.)

of Sand ratio 1.35 1:2.20 1.0 1:2.20 0.5 1:2. 20 1. 1:2.20 1.35 1:2.20 1.5 1:2.20 1.5 1:2.20 1.5 1:2.20 1.5 1:2.20 1.5 1:2.20 1.5 1:2. 20 1.0 1:2.20 1.5 1:2.20 1.5 1:2.20 1.5 1:2. 48 1.5 1:2. 61 1:2.20 1:2. 61 1.5 1:2. 48 1.5 1:2. 61 1.5 1:2. 20 22 CBsPzO: 1.5 122.20 23- CaaP10 1.5 1:2. 48 24 083F101 1.5 122.61 25 Ground phos hate rock 1.5 1:2.20

(-80 mesh) lorida Pebhle). 26- do 1.5 122.48 27.. unfi 1.5 1:2.61 28 Norm 122.20 29 Triphenyl Phosphate 1.0 1.0:2.48 30 Tricresyl Phosphate 1.0 1.0:2.48

! Sintered at 1200 F. Sintered at 1500 F 510% $101, 14% Adzorl-Fe Oz, O3% 08F 2-5% CaCO and 69-78% 083F501.

ammonium phosphates and triaryl phosphates, the amount of said phosphate compound being at least sufficient to provide mold collapsibility after casting and not greater than about 5% by weight of said refractory,

7 about 3 to 15% by weight of said refractory of an alkali metal silicate binder, and impregnating this mixture with a reactant capable of curing the mixture.

2. The method according to claim 1 wherein the silicate is sodium silicate.

3. The method according to claim 1 wherein the phosphate is an alkali metal phosphate.

4. The method according to claim 1 wherein the phosphate is an alkaline earth metal phosphate.

5. The method according to claim 1 wherein the phosphate is finely-divided phosphate rock containing a major proportion of tricalcium phosphate.

6. A composition of matter which consists essentially of a major proportion of a finely-divided refractory material, about 3 to 15% by weight of said material of a sodium silicate binder, and a phosphate selected from the group consisting of alkali metal phosphates, alkaline earth metal phosphates, ammonium phosphates and triaryl phosphates in an amount sufficient to provide mold eo'llapsibility and not in excess of about 5% by weight of said refractory.

7. A composition according to claim 6 wherein the phosphate is an alkali metal phosphate.

8. A composition according to claim 6 wherein the phosphate is an ammonium phosphate.

I 9. A composition according to claim 6 wherein the phosphate is an alkaline earth metal phosphate.

10. A composition according to claim 6 wherein the phosphate is a triaryl phosphate.

11. A metal casting mold comprising a casting-defining structure formed of an intimate mixture of a composition according to claim 7.

References Cited in the file of this patent UNITED STATES PATENTS 1,751,482 Leasman Mar. 25, 1930 2,466,138 Wainer Apr. 15, 1949 2,479,504 Moore et al. Aug. 16, 1949 2,531,496 Bean et al. Nov. 28, 1950 2,680,890 Moore et al. June 15, 1954 2,681,860 Rhodes et al. June 22, 1954 2,749,586 Kohl et al. June 12, 1956 FOREIGN PATENTS 710,099 Great Britain June 9, 1954 OTHER REFERENCES Schumacher: CO Process for Baking Molds and Cores, pp. 46-49, American Foundryman.

Claims (1)

1. THE METHOD OF FORMING A METAL CASTING MOLD WHICH COMPRISES PLACING IN CONTACT WITH A CASTING-DEFINING SURFACE AN INTIMATE MIXTURE OF A MAJOR PROPORTION OF A FINELY-DIVIDED REFRACTORY MATERIAL, A MINOR AMOUNT OF A PHOSPHATE COMPOUND SELECTED FROM THE GROUP CONSISTING OF ALKALI METAL PHOSPHATES, ALKALINE EARTH METAL PHOSPHATES, AMMONIUM PHOSPHATES AND TRIARYL PHOSPHATES, THE AMOUNT OF SAID PHOSPHATE COMPOUND BEING AT LEAST SUFFICIENT TO PROVIDE MOLD COLLAPSIBILITY AFTER CASTING AND NOT GREATER THAN ABOUT 5% BY WEIGHT OF SAID REFRACTORY, ABOUT 3 TO 15% BY WEIGHT OF SAID REFRACTORY OF AN ALKALI METAL SILICATE BINDER, AND IMPREGNATING THIS MIXTURE WITH A REACTANT CAPABLE OF THE MIXTURE.